The present invention is generally directed to tape transport apparatus of the type employed in the data storage industry. More particularly, however, the present invention is directed to air bearings used with such tape transport apparatus which are operative to support a tape on a cushion of air as it is transported through the apparatus, especially across a read/write transducer.
The advent of the information age has seen an exponential growth in the accumulation and storage of data both for on-line usage as well as for archival purposes. In the early days of the computer, before the advent of magnetic disk and optical storage assemblies, data was typically stored on magnetic tapes, such as reel-to-reel tapes and later cassettes. In a magnetic tape storage device, a magnetic coil is used as a transducer to imprint data magnetically on a moving band of magnetic film; thereafter, when the film is advanced across the transducer, the data may be read and re-input into the processor. Magnetic tape has an advantage in that it is relatively low cost and can be erased and rewritten many times. In addition to the medium of magnetic tape, other tape storage media have been developed or are possible. For example, laser written optical tapes may be employed in an effort to increase the density of data stored on the medium.
Magnetic tape is still a highly desirable format for archiving data where the ability to rapidly access the data is of less significance and cost is of concern. The accessibility of data is a function of two variables, the density of storage and the speed at which the tape medium may be transported across and accurately read by the transducer. Here, also, it is important that a lateral edge of the tape be properly registered along a reference plane, called the datum, so that the data may be accurately interpreted.
In any event, where a tape medium is physically moved in a transport direction through a tape transport apparatus, it is necessary both to support the tape during transport and to hold a lateral edge thereof against the datum. Contact of the tape medium with the mechanical parts of the apparatus should be minimized, however, since physical contact between the tape and a mechanical part can cause abrasion of the tape surface or otherwise damage the tape. The elimination of contact is especially important where the tape is under a load at the place of contact. Such abrasions or damage can violate the integrity of the data stored thereon and can sometimes damage parts of the drive mechanism. Thus, it is common to employ a plurality of air bearings in order to support the tape in order to reduce or eliminate friction and contact.
A typical air bearing is in the form of a plenum chamber that has a polished metal bearing surface through which a plurality of ports are drilled. These ports, which are typically on the order of 0.006 to 0.020 inches in diameter, extend as a matrix over the bearing surface and communicate with the interior of the plenum. The plenum may be then pressurized with air which escapes through the ports thereby providing jets of air which form an air cushion to support the tape medium as it travels across the bearing surface. While this type of air bearing is functional, it is not without its disadvantages. First, the manufacture of such air bearings is quite expensive since each of the bearing surface ports is individually, drilled. Thus, for example, over a surface area of 1.5 to 3.0 square inches, it is not uncommon to drill 50 to 200 individual ports having this extremely small diameter. Not only is such drilling time consuming, the extremely small size of the drill bits lead to breakage of the bits. Accordingly, there is a relatively high scrap rate for the air bearings since a broken drill bit may become lodged in a port during the drilling process.
Even where a successful air bearing is formed of this type, it has a relatively limited openness since only approximately 0.1% of the surface is open due to the ports. Therefore, in order to provide sufficient air cushioning force, to resist the tension on the tape as it is transported, the plenum chambers must be maintained at a sufficient pressure, typically on the order of 1.0 to 12.0 psi. Due to compression of the air, heat is generated which heat must be dissipated; otherwise there is a risk of thermal damage to the tape.
In U.S. Pat. No. 5,777,823 issued Jul. 7, 1998 to Gavit, one of the inventors of the present invention, a tape transport apparatus is described. The transport in the ""823 Patent uses an air bearing member which has at least a portion of which is fabricated out of a porous material that allows pressurized air to pass therethrough to produce a relatively uniform cushion of air on which a tape may ride. As described in the ""823 Patent, the air bearing was formed as a housing having an air inlet adapted to be connected to an air source. The housing and the air bearing member are configured to form a plenum chamber having an interior in fluid communication with the air inlet. Alumina was described as the preferred construction of the porous material.
While the air bearing described in the ""823 Patent constitutes a substantial improvement over drilled polished metal air bearings, the construction described therein had some fabrication disadvantages due to the need to seal the porous material at areas where air flow is not desired. Moreover, the mounting of the porous bearing in the metal housing creates some difficulties in mass production. Finally, like other air bearings of the drilled, polished metal type, this air bearing had undesirable variations in the wrap angle for a tape moving off of the portion of the air bearing remote from the recording head and onto a respective take-up or supply reel. This variable wrap angle was known to be a lesser problem for non-air bearing transportation systems utilizing small diameter rollers. These systems, though, have a substantial disadvantage in that the inertia of the rollers are a limiting factor in the rapid acceleration and deceleration of the tape medium; in addition, there is the likelihood of more contact between the tape and the transport rollers.
As noted above, it is also necessary that one lateral edge of the tape be held against the datum so that data may be accurately read or written by the transducer. In the past, this technique has been accomplished by the use of a plurality of spring fingers which carry, at their distal ends, lubricous buttons which bear against a second lateral side edge of the tape, opposite the first lateral edge which is to be held against the datum. While this technique of physically biasing the tape against the datum has been successful, it still leads to undesired contact of the tape medium as it is transported at high speeds across the air bearing. This can cause excessive friction, and the mechanical biasing of the tape can cause damage to the tape edge. Moreover, the tape edge can eventually create undesired grooving of the buttons. In the ""823 Patent, the biasing of the lateral edge of the tape against the datum was alternatively accomplished by angling the bearing surface to the datum or by the conventional spring fingers.
Also, in air bearing systems, there are always two locations, i.e., at the opposite ends of the bearing surface, where the tape approaches and departs the bearing. At these locations, the flying height of the tape, that is, the separation distance between the tape and the bearing surface is less at the edges than at the fully wrapped region between the ends. In this fully wrapped region, the pressurized air can escape only at the tape edges. At the departure locations, pressurized air will escape as well. Since there is a loss of support from the air film at these locations as the positive pressure reduces to ambient, tape debris can often accumulate and possibly damage the recording medium. Such debris may also clog the pores of the traditional drilled air bearing. Such problems are exacerbated by the rapid advancement of the tape off of the bearing as the moving tape drags the air forming the cushion off of the bearing surface.
Accordingly, there remains a need for improved tape transport apparatus and, more particularly, for improved air bearings which can support a tape medium during rapid transport through such an apparatus. There is also a need for air bearings that eliminate the disadvantages attendant the individual drilling of ports through the bearing surface in order to provide air jets to form an air cushion that supports the tape. The present invention is directed to meeting these needs.
An object of the present invention is to provide a tape transport apparatus incorporating improved air bearings therefore which air bearings are constructed to provide an air cushion that supports a taped medium during transport.
Another object of the present invention is to provide an air bearing that reduces problems associated with variance of the wrap angle of the tape near the supply cartridge and near the take-up reel.
Still another object of the present invention is to provide porous air bearings that have better datum tracking capabilities.
A further object of the present invention is to provide a porous air bearing for tape transport systems that eliminates disadvantages of initial air build-up as well as departure tape sag.
It is yet another object of the present invention to provide a new and useful method of fabricating porous air bearings which method is simple and economical.
Still a further object of the present invention is to provide a method of manufacturing porous air bearings that provides a simplified sealing technique for surfaces through which air is not desired to pass.
According to the present invention, then, an air bearing system and a tape transport apparatus utilizing such air bearing system is provided. The air bearing system is adapted to support a tape on a cushion of air as the tape is transported in a transport direction thereacross. In its broad form, the air bearing system includes a air bearing member that has an air bearing surface with a first surface portion and a second surface portion. A source of pressurized air communicates with the air bearing member. The air bearing member is constructed such that air escapes through the first surface portion at a greater pressure than air that escapes through the second surface portion.
The air bearing system according to one embodiment of the invention includes a plenum associated with the air bearing member with this plenum being in communication with the source of pressurized air. The air bearing member then has a wall with one surface thereof in fluid communication with the plenum and an opposite surface thereof defining the air bearing surface. This wall is fabricated out of a porous material. A first portion of the wall underlying the first surface portion of the bearing member is thinner than a second portion of the wall underlying the second surface portion. Preferably the first surface portion is formed at a first radius of curvature and the second surface portion is formed at a second radius of curvature that is larger than the first radius of curvature. This results in an arcuate air bearing surface that has a dual radius of curvature. The first surface portion may extend for approximately 45 degrees of arc while the second surface portion can extend also for about 45 degrees of arc.
Alternatively, the air bearing member can include separate first and second plenums with the first plenum being associated with a first surface portion and a second plenum being associated with the second surface portion. The first and second plenums may be separated by a dividing wall of porous material so that a single source of pressurized air may be directly connected to the first plenum to supply air at the first pressure with the second plenum then being pressurized by air passing from the first plenum into the second plenum through the dividing wall. This dividing wall is fabricated of a porous material selected from a group consisting of ceramics, metals and composite materials. Alternatively, separate and distinct first and second plenums may be provided with the source of pressurized air including distinct first and second air supplies having different pressures so that one air supply is in fluid communication with the first plenum and a second air supply is in fluid communication with the second plenum.
The air bearings according to the invention may include a rib disposed at one or both ends of the air bearing surface. Such rib extends transversely across the air bearing surface and extends upwardly from the air bearing surface selected distance in order to resist flow of air across the rib as the tape moves thereover. These ribs may have an upper curved surface or a flat surface, with the latter being preferred. In either event, such ribs act as dams to prevent leakage and the drag off of air as the tape advances onto and off of the air bearing surface.
Moreover, the air bearings of the present invention include a datum face operative to define a guide surface for first lateral edge of the tape. The datum face is oriented in a datum plane that is parallel to the transport direction. Here, the air bearing surface is oriented at large acute angle with respect to the datum face with this angle being within a range of 70 degrees to just slightly less than 90 degrees, but preferably about 88.8 to 89.3 degrees.
The present invention also is directed to a tape transport apparatus that transports a tape in a tape drive in a reversible transport direction between a supply reel and a take up reel across a recording head. The tape transport apparatus includes a first air bearing member disposed on one transport side of the recording head and a second air bearing member disposed on another transport side of the recording head. Each of the first and second air bearing members then includes an air bearing surface having a first portion formed at a first radius of curvature located proximate to the respective supply and take up reels and a second surface portion formed at a second radius curvature located proximate to the recording head. The first radius of curvature is smaller then the second radius of curvature. Each of the air bearing members are constructed so that air will pass through the first and second surface portions to produce of cushion of air on which the tape may ride. A source of pressurized air is in communication with the first and second air bearing members with the first and second air bearing members constructed such that air escapes through the first surface portion at a greater pressure than air that escapes through the second surface portion.